more radio antennae

7/28/2019 More radio antennae

1/5

October 1998 45

Ihave had the pleasure of hearing thewell-known antenna master, LewMcCoy, W1ICP, half jokingly ma-lign the reputation of the vertical

antenna. Although there is some uncer-tainty about who actually was the first tostate that vertical antennas radiate equallypoorly in all directions, Lew did make thisstatement in his antenna presentation at the1996 ARRL Southwestern Division Con-vention. Well, despite this folklore, I havealways found vertical antennas to be sur-prisingly good performers. Consideringtheir simplicity, it is hard to beat them aslow-cost DX antennas. In my personal ex-perience, they often defy their reputationfor poor performance.

The antenna described here is a versa-tile, multiband vertical designed for use on160, 80, 40 and 20 meters. My goals wereto produce a reduced-size design thatwould fit on a modest lot, could be erectedby just a few people, has maximized effi-ciency and costs no more than $200. Evenif you decide not to build the antenna asdescribed, you can apply the principles tosingle-band antennas or scale the antennafor the 80 through 10-meter bands.

Figure 1 shows the major points of the

design. The main support is a Rohn 50 tele-scoping mast, commonly used for fringe TVinstallations. Fully extended, the mastheight is a little over 44 feet; it serves as thevertical element on both 160 and 80 meters.With the help of two short cross arms, themast also supports vertical wires for 40 and20 meters. The remainder of the antennaconsists of two top-hat wires that addcapacitive loading on 160 and 80 meters.On 80 meters, a pair of parallel-resonanttraps disconnect some of the 160-meter tophat to provide appropriate loading. This isthe only use of traps in the antenna, and Ivemade every effort to minimize their loss.

Alternatively, the mast can be made

By Thomas Kuehl, AC7A

An Efficient Multiband

Vertical for 160 through20 Meters

1Notes appear on page 49.

from many different materials, such asaluminum tubing, military-surplus cast-aluminum mast sections, tower sections ora wire hung from a conveniently placedtree. The primary effects of such substitu-tions will be variations of the wire lengthsshown in Figure 1.

The operation on each band may not becompletely evident, so a band-by-band ex-planation may help:

160 Meters

The 44-foot mast is well short of the

132 feet required for /4 resonance on160 meters; therefore, loading is required toestablish resonance. Rather than resort toinductive loading, with its inherently highlosses, I incorporated a low-loss capacitancehat. A folded top hat design provides theadditional capacitance necessary for reso-nance and requires less horizontal spacethan a flat top hat. The top hat approach alsoincreases the radiation resistance, whichimproves the antennas efficiency.1

With a vertical length less than 0.1 on160 meters, this antenna systems total re-

sistance (RA) will be lowusually in therange of 5 to 25 . RA is the combination ofradiation resistance (RR) and ground-returnresistance (RG). The final RA value dependsheavily on the physical installation of thetop hat and the value of RG

(see Note 1). On

this band, a lower RA indicates that systemlosses are low, and one can expect reason-able efficiency.

In order to achieve a match to a 50-feed line, a matching transformer with somelatitude is required. Figure 2 details a broad-band transformer that allows a number of

impedance-matching ratios; one of themshould satisfy the RA at your particular in-stallation. Should a higher impedance ratiobe required (high RA), make taps in the pri-mary winding.The figure also shows howan SPDT relay can switch out the trans-former secondary on 80 through 20 meters.RA is already close to 50 on these bands,so a transformer usually isnt required.Leaving the transformer primary in the cir-cuit on other bands has no adverse affect,and provides a direct path to ground forstatic discharge. The dc relay voltage canbe supplied by a separate cable, or throughthe feed line if isolation chokes and capaci-

tors are used.

Build this low-loss,low-cost antennafor the low bands.


2/5

80 Meters

A /4 vertical on 80 meters ought to beabout 66 feet in length. Therefore, themasts 44-foot length fulfills two-thirds ofthe antennas requirement. By trapping off

most of the top hat, the remainder is used to

Figure 1A diagram of AC7As multiband vertical antenna system.

Figure 2A circuit to switch in a multitap-matching transformer

for 160-meter operation. T1 is an FT-140-61 core with 12 bifilarturns of #15 AWG (see text) Polythermaleze wire. Arrange thebifilar wires side-by-side; do not twist them. Elevate turns 6through 11 of the secondary above the toroids outer face tofacilitate taps. K1 is a surplus relay (12 V, with 15 A contacts).Mouser stocks a suitable relay (Magnacraft & Struthers-Dunn;W78RCSX-97, Mouser #528-7880-97) You can contact Mouserat 958 N Main St, Mansfield, TX 76063-4827; tel 800-346-6873;e-mail [email protected] , http://www.mouser.com. [Theauthor has used this relay and wire with 150-W output.Ed.]

Figure 3A homebrew spacer relieves strain from the seriescapacitor at the bottom of the 40 or 20-meter wire elements. Forpermanent installation, this must be weatherproofed.

establish resonance on 80 meters. The trapswere designed as high-Q circuits to mini-mize losses.

By inclusion of the traps on 80 meters,the small, finite loss resistance of the traps

is transformed to a larger value at the feed

point. This small resistance, in conjunctionwith RR and RG, produces a good match toa 50- feed line.

40 and 20 Meters

Short cross arms at 2 and 39 feet above46 October 1998


3/5

the antenna base support the 40 and 20-meter antenna wires and space them awayfrom the mast. The physical spacing re-duces tuning interaction between bandsand helps keep the wires out of the way

when the mast is being extended or col-lapsed. They are joined at the bottom of themast, which serves as a common feed point(see detail in Figure 1).

The 40 and 20-meter antenna wireswere intentionally made long. This adds in-ductive reactance that can be resonated bya series capacitor. This not only allows theantenna to be tuned at the base, but alsoeliminates the need to raise and lower themast while adjusting the antenna for thesebands. It also raises the RA over that of a/4 vertical, which improves efficiency andprovides a good match to a 50- feed line.

The 40-meter vertical wire is 39 feet long,

and the 20-meter vertical is 18.5 feet long.Both are opened about six inches from thebase connection point to insert the series ca-pacitors. When tuned, they provide an excel-lent match to a 50- feed line. Systems withvery low ground losses (RG < 5) can achievea radiation efficiency on the order of 90%.

Construction

There are no steadfast rules in the con-struction of a homebrew antenna; noteveryone has access to the same materialsand tools. Therefore, I encourage you to becreative in making do whenever necessary.

As a safety consideration, assess yourphysical ability to erect the telescoping mastbefore attempting to do so. When in place,the top end of the bottom section will bemore than eight feet above the ground. Eachmast section must be pushed up and lockedinto position while working from this height.This is best accomplished by two people,working from two tall ladders. New mastsections slide easily, but with time, corro-sion and dirt buildup make the task moredifficult. Keep the mast sections clean andlubricated to help with future disassembly.Aside from extending and collapsing themasta physically tiring actnothing ex-

traordinary is required to erect this antenna.

The Rohn 50 telescoping mast can bemail ordered from many Amateur Radiooutlets, but be prepared for shipping feesthat nearly equal the mast cost. With a littlesearching, I was surprised to find that they

are stocked by a local electronic supplierat a cost comparable with the mail-orderdistributors.

The two cross arms were made from asingle five-foot piece of antenna mast, thethin-wall type used to support TV anten-nas. The swaged end was removed witha hacksaw and discarded. The remainderwas cut into two equal-length pieces, about27 inches long. A widely spaced U bolt(2 11/16 inches) fits around the lower mastsection and through holes drilled for it atthe center of the cross arm.

If theyre available, use U-bolt clampsto help mount the upper cross arm. They

also help keep the cross arms fromwindmilling when the wires are pulled.Since the lower cross arm does not carry aload like the upper cross arm, I didnt use aclamp, and it hasnt presented a problem.

Holes were drilled 1.5 inches from thecross arm ends to facilitate installing eyebolts through the top cross arm and to passthe 40 and 20-meter wires through on thelower cross arm. Remember that theseholes are drilled at 90 to the U-bolt holes.

The 40 and 20-meter wire elements fas-ten to the upper-cross-arm eyebolts withDacron rope. An egg insulator is used be-tween the wire and rope. A short piece ofrope is sufficient for the 40-meter wire;about 20 feet supports the 20-meter wire.Lay the extended mast on the ground be-fore installation to set up the cross-armpositions and measure the exact ropelengths required to support the wires.

I mentioned that the 40 and 20-meterwires are spaced about six inches from themast at the antenna base for placement ofthe series tuning capacitors. The capacitorleads will not withstand the wire tension,but mounting the capacitor across an egginsulator solves the problem. Alternatively,make an appropriate insulator from a piece

of acrylic plastic mounted between two

(A)

(B)

Figure 4A is a cross section of the traps. B is a photo of the finished trap. Notice the ring terminals on the top-hat wires and thecable ties used for strain relief.

studs. Some form of weatherproofing isnecessary. See Figure 3 for details of ahomemade insulator.

The wire used for the top hat and verti-cal wires was purchased from Antennas

West.2

It is sold under the name Quiet-Flex. It is a durable, 40-strand, #14, 3500-V-insulated copper wire, with a black, UV-resistant jacket. Other #14 insulated wirewill work, but Quietflex will stand up tomost any kind of sun or weather conditionfor a long time. It is flexible at most anytemperature and is a pleasure to use. Toprotect it as it passes through the lowercross arm, I slipped a four-inch piece ofRG-58 outer jacket over the wire to protectit from chafing3/16-inch-diameter heat-shrink tubing should work as well. Just re-member to slip it over the wire before mak-ing any permanent connections.

All antenna-wire connections weremade using crimped and soldered ring ter-minals that fit #10-24 screws. Each termi-nal ring is sandwiched between a pair ofwashers and nuts at each connection. I usedheavyweight ring terminals and find thatthey take much more punishment than theirlightweight counterparts.

The top-hat wires can be connected tothe top of the mast in several ways. Youcould drill two opposing holes about one-half inch from the mast end and install a#10-2411/2-inch-long machine screw ineach hole, with the threaded end pointingoutward. Secure the ends of the top-hatwires to these studs using #10-24 hardware.

I mounted the studs in a piece of heavy-gauge PVC. It is slit at one end, slippedover the end of the mast and held in placeby a stainless-steel hose clamp tightenedover the slit end. The studs connect to themast via short jumper wires fabricatedfrom antenna wire and ring terminals. Aself-tapping screw in a drilled pilot holeholds each terminal to the mast.

The top hat is folded back upon itself byrunning the wire through a small pulley atthe fold point. This allows the fold-backpoint to move as necessary for each unique

installation. Since the wire is insulated,

October 1998 47


4/5

either a metal or plastic pulley can be used.The free end of each wire terminates at aninsulator. The wire insulators are con-nected, one to each end, of a six-foot ropethat slides through a third insulator. Thatthird insulator is attached to the guy ring atthe top of the second highest mast section(see Figure 1).

Place the top-hat fold points as high aspossible; a horizontal installation is opti-mum. Higher top hat positions increase the

RR portion of RA on 160 and 80 meters,which helps maximize efficiency. Shouldtall trees or other structures be available,they will work as end supports. There arefew tall trees here in the desert, so I usesome thin-wall mast sections. In my instal-lation, a 10-foot mast was used at each end.Since my lot is long, the masts are spacedabout 30 feet from the fold points. Thispositions the top hat ends about 18 feetabove ground.

It requires some ingenuity to hold themast in place and insulate it from theground. I buried the bottom half of a cham-

pagne bottle in the ground, centered themasts bottom section over the bottle andtightly guyed the mast with galvanized steelwire. Egg insulators isolate the mast andguy wires. The galvanized wire has lessgive than rope and provides for more rigidmast placement.

Dacron rope was used for all the mastsections above the bottom section. Al-though you could guy the mast at each tele-scoping section, I installed guys only at thebottom, center and top sections. The mastbears no load at the top, and the top hatserves to guy the antenna. This approachhas worked well, and its much less confus-

ing when the guys and various wires arebeing pulled into position. The antenna haswithstood strong winds and stayed up with-out any problems.

The ARRL Antenna Book shows manysuitable guy-anchor systems.3 I use three-foot construction stakes obtained at a localhome-improvement center. They are strongsteel stakes that taper to a point at one endand have three convenient holes throughthem right near the top. Rather than thread-ing the Dacron rope directly through thestake, I slip a piece of galvanized guy wirethrough the hole and twist it on it self. Theguy then terminates in an egg insulator that

provides a good union for the Dacron rope.The guy wire is several feet long to keep theDacron rope above ground and away fromrabbits that might chew through it.

One last point: As with any vertical an-tenna, the performance of this antenna ishighly dependent on the ground conductiv-ity near the antenna. The usual method ofimproving the ground conductivity is touse many radial ground wires around theantenna base. This and other possibilitiesfor developing a ground system are cov-ered in The ARRL Antenna Book.4 Also,refer to the sources described in end notesof the authors previous articles on build-

ing efficient, short vertical antennas.Trap Construction

The trap construction is shown in thedrawing and photograph in Figure 4. Black17/8-inch (OD) ABSserves as the coil form.The coil windings are secured at the endsusing studs made from #10-2411/2 screws,spaced 11/2 inches apart. I used brass hard-ware, but stainless-steel hardware would bebetter, where corrosion is a problem. Eachcapacitor is housed inside its coil form andconnects under the screw heads with ringterminals.

Before winding the coil, prepare the ca-

pacitor for installation inside the form. A500 pF, 2500 V mica capacitor is required.Traps must withstand high voltages and a2000 V to 2500 V rating should be sufficientfor power levels in the 100 to 150 W range.If you have problems coming up with high-voltage capacitors, consider connecting sev-eral lower-voltage capacitors in series, per-haps four 2000 pF, 500 V silver-micacapacitors. When series connected, they areequivalent to a single 500 pF, 2000 V ca-pacitor. Silver-mica capacitors have lowdielectric loss, tight tolerances and are avail-able from a number of electronic supplierswho carry surplus or new components.

The coils are made from nine turns of#15 AWG enameled wire (#14 or 16 AWGshould also work), and should have a fin-ished turn spacing of about eight turns perinch. Strip the insulation off one end of thewire and anchor the wire to a stud. Windthe coil tightly and closely space the turns.Strip the free wire end and secure it to theother stud.

The coil turns will be spread during thetuning operation. As the turns are spread,the windings may loosen. If this happens,simply grip a turn with some pliers andtwist it to form a wire kink on the formsurface. This will tighten the coil winding;

repeat the action for more turns until thewinding is tight.ABS end caps help keep water out of the

trap. Connect the top-hat wires to the coil-end studs with heavy-duty ring terminals.(See Figure 4B.) Secure each top-hat wireto the trap end caps with a heavy-duty, UV-resistant nylon cable tie to reduce the loadon the ring terminals.

Trap Tuning

Before venturing outside, the traps mustfirst be tuned. There are several ways to dothis using a dip meter or one of the variousantenna-measurement devices offered by

MFJ or Autek. I use a Heathkit dip meter inconjunction with my station transceiver toget an accurate frequency indication.

You must decide where in the 80-meterband you want to tune the antenna and traps.Since I operate mostly at the low end of80 meters, I tuned the traps to 3.530 MHz.When the coil turns are close up againsteach other, the trap resonates below the80-meter band. As the turns are spreadapart, however, the resonant frequencyrises. Use a pocketknife and work towardan even spacing of about 1/8 inch betweenturns. Spread and compress the windingsuntil you achieve the desired resonant fre-

quency. Once the tuning is complete, applya few lines of epoxy lengthwise, across thewindings, to hold them in place. After theepoxy cures completely, the traps may beinstalled in the top-hat wires.

Antenna Tuning

Antenna tuning can be accomplishedusing a low-power transmitter and an SWRbridge, or one of the mentioned antenna-measurement instruments. For 80 through20 meters, no impedance matching is re-quired: Simply adjust the appropriatewires length or change the capacitor value.For 160 meters, both the top hat length and

Figure 5An SWR versus frequency chart for the multiband vertical antenna.

48 October 1998


5/5

feed-point matching must be set.Begin tuning on 80 meters. Adjust the

wire length (between the top of the mast andthe traps) until resonance is achieved. A lowSWR should accompany the resonant point.Depending on the desired frequency and theenvironment, the wire lengths should benear those shown in Figure 1. Start with awire that is a little too long to avoid splicingwires. For my installation, the mast plus80-meter top hat lengths are:

L (ft) = 230 / f (MHz) (Eq 1)

The required length will be shorter if thetop hat is installed horizontally.

Once satisfied with the 80-meter tun-ing, adjust the overall top hat length forresonance on 160 meters. This is accom-plished by shortening or lengthening thelong, folded wire beyond the traps. Asmentioned earlier, the RA on 160 meterswill be far less than 50 , which may makeit difficult to find the initial resonant fre-quency with an SWR meter. I suggestconnecting the matching transformer of

Figure 2, with the 12.5-

tap selected. Thisshould provide a reasonable match for mostinstallations and allow the minimum-SWRpoint to be determined. The optimum tapselection can be made after the antenna isresonant at the target frequency. For160 meters, the total length of the mastplus the full top hat, followed the equation:

L (ft) = 255 / f (MHz) (Eq 2)

If you must add long lengths to the160-meter top hat, the folded ends are tooclose to the ground.

A space is intentionally provided be-tween the top-hat wire ends and the mast by

the three-foot ropes shown in Figure 1. Thishelped reduce a peculiar end effect thatmade tuning less predictable. Thereforewhen adjusting the top hat length, changethe end of the wire, but do not reduce thewire-to-mast spacing.

The 40 and 20-meter bands are eachtuned in similar fashion and do not requireraising and lowering the mast. Figure 1shows approximate values for the seriestuning capacitors: 200 pF for the 40-meterelement and 150 pF for the 20-meter ele-ment. These values provide resonance inthe CW subbands, with good bandwidth.You can easily determine exact values by

substituting a variable capacitorsuch asa 365-pF receiving capacitorfor the fixedvalue. Using an insulating knob on the ca-pacitor, adjust it until the SWR minimum isat the chosen frequency. Remove the ca-pacitor from the circuit, measure its valueand replace it with the closest availablefixed-value capacitor. A silver-mica ca-pacitor works nicely in this application, andsince it is used at a low-voltage point, itneed not have a high working voltage likethose of the traps.

Once you are satisfied with the tuningon each band, secure the guys and endropes.

Results

The 2:1 SWR bandwidth of an antenna isalways of interest. It doesnt indicate radia-tion performance, but rather the frequencyrange that can be covered without using aTransmatch. Wide SWR bandwidth can alsoindicate excessive loss, so keep this in mind.Figure 5 shows SWR plots for each of thebands. The curves are based on measure-ments made at the antenna feed point.

Just for the sake of curiosity, I checkedthe SWR on each of the other HF bands, aswell. A couple of surprises did show up:The SWR on 30 meters is a little less than2:1, and its slightly greater than 2:1 in theFM portion of 10 meters. Although it wasnot intended for 30 meters, the antennaperforms well on the band.

The proof of any antenna design comesdown to how well the antenna performs atyour particular station, with its particularconstraints. I use 16 buried radials at thislocationranging in length from 66 to 130feet. This is a minimal ground system, butthe antenna is located in an area covered

with dense desert vegetation and it pre-cludes the installation of a more extensivereturn system. Despite this limitation, theantenna does a credible job. To date, Iveworked all states and continents, exceptEurope, on 160 meters, using 80 to 90 W onCW. The DX performance has been evenbetter on 80 meters, where stations on allcontinents have been contacted. Using thislatest version and various previous genera-tions of verticals, I have managed to workDXCC on 40 meters with an output power

less than 100 W. Thats not a bad accom-plishment for an antenna that radiatesequally poorly in all directions.

Notes1Thomas Kuehl, AC7A, Build Efficient, Short

Vertical Antennas, QST, March 1998.2Antennas West is no longer in business. An-

tennas and More sells all Antennas Westproducts. Contact them at 1038 S 350 E,Provo, UT, 84606; tel 888-277-5718, 801-

373-8426 (both lines voice and fax); [email protected]; URLwww.antennasmore.com.

3R. Dean Straw, N6BV, Editor, The ARRL An-tenna Book(Newington: ARRL, 1997), ARRLOrder No. 6133. See Chapter 22. ARRL pub-lications are available from your local ARRLdealer or directly from the ARRL. Mail ordersto Pub Sales Dept, ARRL, 225 Main St,Newington, CT 06111-1494. You can call ustoll free at tel 888-277-5289; fax your order to860-594-0303; or send e-mail to [email protected]. Check out the full ARRL publicationsline on the World Wide Web at http://www.arrl.org/catalog.

4R. Dean Straw, N6BV, Editor, The ARRL An-tenna Book; see Chapter 3.

Thomas, AC7A, was first licensed in 1969 at age

15, with the call WN6BQI. Early on, he developedan appetite for the technical side of the hobbythat remains today. A graduate of Cal-Poly SanLuis Obispo, he has spent the better part of the

last 19 years employed by Burr-Brown Corpora-tion as a Product Engineer. When not assistinghis wife with raising their three children, youmay find him chasing DX on the low bands orpruning antenna wires in the backyard. You can

reach Thomas at 620 S Avenida Princesa, Tuc-son, AZ 85748; e-mail [email protected].

New ProductsGIANT LCD STATION CLOCKFROM MFJ

Visible from across any room, MFJsGIANTdisplay jumbo LCD station clockhas 2.25-inch high-contrast digits and dis-plays time in 12- and 24-hour formats, alongwith year, month, date, day and indoor

MOUNTAINEER FIBERGLASS BASE

STATION ANTENNAS FROMANTENEX

The newest addition to the Antenex lineis the Mountaineer base station vertical, adurable, lightweight antenna that features50-year weather seals, aluminum and stain-less steel hardware, flexible mounting op-tions and an innovative radial system.Available in models from 150 to 970 MHz,the Mountaineer can be mounted conven-tionally or inverted. For pricing and addi-tional information, contact Antenex, 2000-205 Bloomington Rd, Glendale Heights, IL60139; tel 800-323-3757; fax 800-851-

9009; http://www.antenex.com.

temperature. The quartz-controlled time-piece is housed in a scratch-resistant plasticcase (8.5 9 inches), has mounting holesfor convenient installation and uses a stan-dard AAA cells for power.

Price: $49.95. The GIANTdisplay clockis backed by MFJs No Matter What one-year limited warranty. For more informa-tion, see your local Amateur Radio prod-ucts dealer or contact MFJ, PO Box 494,Mississippi State, MS 39762; tel 800-647-1800; fax 601-323-6551; http://www.mfjenterprises.com.

October 1998 49

more radio antennae

Documents